If you dd from sdcard to emmc keep your size below 14.5g or your emmc will not boot, don't ask how i know this.
Or label your sdcards , and know what is on them.
Its easier to adjust the size with gparted or similar before you “ dd ” it.
Update, finally got round to trying to import the gpx to opentopomap.org directly to morph...works! But performs very sluggishly on my N5. As an alternative, Activity Tracker seems to make trace files ok without draining battery too much and I can import other people's gpx files there too...so problem solved.